1. Field of the Invention
This invention relates to current leads for conducting current to or from superconducting equipment contained in a cryostat. More particularly, it relates to such power leads incorporating composite high temperature, superconductors.
2. Background Information
Current leads transmit power between cryogenic equipment located within a cryostat and a power supply or load located at higher temperatures, such as room temperature. Conventional current leads use metals such as copper for their entire length. These leads introduce heat leak to the cryostat as a result of heat conduction from the external conductors and the resistive heating in the lead itself. It is generally recognized that the current lead is an important if not a dominant source of heat leak into the liquid cryogens such as liquid helium. The heat leak causes the cryogen to boil off and, for an open cycle system sets a limit on the operating time of the cryogenic equipment. For a closed cycle system, a refrigeration and liquefier system is needed to recondense the cryogen vapor back to its liquid phase. Due to the temperature difference and limitations of liquefier efficiency, the refrigeration power required to recondense the cryogen vapor back to the liquid state generally is several hundred to over a thousand times the heat leak to the cryogen pool. A substantial reduction in refrigeration system capital cost as well as the operating cost can therefore be achieved by the reduction of heat leak to the cryogen pool.
The heat leak can be reduced by minimizing the resistive heating and/or heat conduction. The newly discovered ceramic superconductors, such as Y--Ba--Cu--O (YBCO), Bi--Sr--Ca--Cu--O (BSCCO) and Tl--Ba--Ca--Cu--O (TBCCO) systems, have critical temperatures higher than liquid N.sub.2 temperature and can thus be used to eliminate the resistive heating in the low temperature part of the lead. These ceramic materials also have a thermal conductivity significantly lower than copper at temperatures near the liquid helium temperature. Both of these properties are ideally suited for minimizing the heat leak in the current lead.
The feasibility of this concept has been successfully demonstrated by a 2-kA current lead described in U.S. patent application Ser. No. 07/585,419 filed on Oct. 20, 1990. Heat leak reduction of up to 40% from optimized conventional leads has been achieved in testing.
The basic feature of the lead described in that application for achieving low heat leak is the employment of an array of ceramic superconductor bars which may, for example, be made from powders of YBCO and silver (15% vol.). These bars are connected to the cryogenic device via a lower copper plate and are designed to operate from 4.2K to liquid nitrogen temperature. An array of copper conductors operates at a higher temperature range and interfaces to the room temperature power supply.
The superconductor bars are fabricated by pressing/sintering/annealing method and, unless time consuming and expensive melt texturing technique is applied, generally have low critical current density of 100-300 A/cm.sup.2 at zero external magnetic field and decreases rapidly with increasing external field. The limitation of critical current density produces two drawbacks for current lead application. First, the aforementioned superconductor current density is lower by a factor of five to ten as compared to the current density generally used in the copper portion of the lead. Thus, the superconductor portion requires substantially larger cross sectional area than the copper portion. This introduces complications in the design and fabrication of the lead and may exclude its use from the cases in which the space available for lead installation is not sufficient for the superconductor portion of the lead. Secondly, the low current density allows only limited margin in design flexibility in the considerations of conductor stability and heat leak optimization of the lead.
Composite superconductors of high critical current density have been developed such as silver sheathed composite high-T.sub.c superconductor wire made from powder-in-tube technique. Critical current densities as high as 31,000 A/cm.sup.2 at 77K and 0.1T, and 11,000 A/cm at 77K and 1.0T have been demonstrated in silver sheathed Bi-based superconductor tape-shaped wires. In this kind of conductor, the silver sheath serves as a stabilizer as well as a mechanical structure to confine and strengthen the ceramic superconductor material. However, due to high thermal conductivity at low temperature, the silver sheath can introduce significant heat leak to the cryogen.
U.S. Pat. No. 4,895,831 discloses a cryogenic current lead having a ceramic superconductor wound on the sleeve of a cryo-cooler to increase the overall length of the lead and therefore reduce heat conduction through the lead.
There remains a need for a power lead for cryogenic equipment which has a high current density capacity yet contributes a minimum to heat leak into the cryostat.